This invention relates to cardiac monitoring and more particularly to a method and apparatus for determining cardiac volume.
It is highly beneficial to monitor in real-time, the beat-to-beat stoke volume, ejection fraction and ventricular pressure-volume loops in patients undergoing cardiac surgery. Monitoring these parameters allows for better evaluation of the physiological status of these patients.
There are several known methods of measuring cardiac output including echocardiography, contrast angiography and conductance catheter method. Of these, the conductance catheter method can potentially measure the absolute ventricular volume continuously in real time. Conductance catheters may comprise a modified Swan-Ganz catheter, for example, with a set of equidistant, metallic ring electrodes at the distal end. The Catheter is inserted into the patient's ventricle and a constant electrical current having a fixed frequency is applied to spaced apart exciting electrodes to establish a constant-current electric field within the blood volume of the ventricle. The spaced apart electrodes between the exciting electrodes (sensing electrodes) divide the ventricular volume into parallel segments as shown in FIG. 1. Voltage signals at pairs of spaced electrodes disposed between the exciting electrodes are sampled. This provides a potential drop (V.sub.i) across these volume segments. Knowing V.sub.i and the constant current (I) injected by the exciting electrodes, the resistance across the volume segments (R.sub.i) can be computed as follows: EQU R.sub.i =V.sub.i /I (1)
The segmental resistances can be related to the volume of each segment by the expression: EQU Vol.sub.i =.rho.L.sup.2 /R.sub.i ( 2)
where
Vol.sub.i =Volume of the i.sup.th segment, PA1 .rho.=blood resistivity, PA1 L=spacing between the electrodes, and PA1 R.sub.i =Resistance of the i.sup.th segment. PA1 1. Electric field non-uniformity. Equation (2) above is based on a uniform electric field. In actual practice, the electric field created by a conductance catheter is not uniform, and this causes the measurement of segmental volume to be inaccurate. PA1 2. Leakage of current through surrounding tissues. Some of the injected conductance catheter current leaks out through the surrounding tissues such as lung and cardiac tissues so that the electric field is no longer confined to the blood volume of the right ventricle. This also introduces errors in the measurement of segmental resistances. PA1 3. Catheter curvature. The catheter assumes a curved orientation in the right ventricle as shown in FIG. 2. The curved orientation of the catheter coupled with the non-uniformity of the electric field results in erroneous measurement of segmental volumes. PA1 4. Blood resistivity changes. Blood resistivity changes primarily due to changes in hematocrit, temperature and blood velocity, which are not uncommon in cardiac patients undergoing surgery. Hence, measurement errors can result if blood conductivity changes are not monitored.
Conductive catheter methods of cardiac output measurements are disclosed in U.S. Pat. Nos. 4,674,518; 4,721,115; 4,898,176; 4,491,682, and 5,000,190. A method of determining blood resistivity is disclosed in co-pending application Ser. No. 08/701,177, filed Aug. 2, 1996, which is incorporated by reference herein.
From the segmental volumes, the total volume can be determined from the expression ##EQU1## where n=total number of volume segments.
Prior attempts to measure ventricular volume with the conductance catheter have mostly been confined to the left ventricle. Under clinical conditions, however, measurement of the right ventricle would be preferable, because the right side of the heart is more easily accessible than the left side. Also, the lower pressure in the right side of the heart and the greater clinical experience with right-side catheterization make it less dangerous to use the catheter on the right side compared to the left side. For these reasons, it is desirable to provide a conductance-catheter measuring method and apparatus for measuring right-ventricular volume.
Various studies have shown that there are four main sources of error in conductance-catheter measurements. These are: